{
 "cells": [
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "$$\n",
    "\\def\\CC{\\bf C}\n",
    "\\def\\QQ{\\bf Q}\n",
    "\\def\\RR{\\bf R}\n",
    "\\def\\ZZ{\\bf Z}\n",
    "\\def\\NN{\\bf N}\n",
    "$$\n",
    "# Using the Python Interpreter\n",
    "\n",
    "## Invoking the Interpreter\n",
    "\n",
    "The Python interpreter is usually installed as /usr/local/bin/python3.8 on those machines where it is available; putting /usr/local/bin in your Unix shell's search path makes it possible to start it by typing the command:"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "python3.8"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "to the shell.[1] Since the choice of the directory where the interpreter lives is an installation option, other places are possible; check with your local Python guru or system administrator. (E.g., /usr/local/python is a popular alternative location.)\n",
    "\n",
    "On Windows machines, the Python installation is usually placed in C:\\\\\\\\Python36, though you can change this when you're running the installer. To add this directory to your path, you can type the following command into the command prompt in a DOS box:"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "set path=%path%;C:\\python36"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "Typing an end-of-file character (Control-D on Unix, Control-Z on Windows) at the primary prompt causes the interpreter to exit with a zero exit status. If that doesn't work, you can exit the interpreter by typing the following command: `quit()`.\n",
    "\n",
    "The interpreter's line-editing features include interactive editing, history substitution and code completion on systems that support readline. Perhaps the quickest check to see whether command line editing is supported is typing Control-P to the first Python prompt you get. If it beeps, you have command line editing; see Appendix [tut-interacting](tut-interacting.ipynb) for an introduction to the keys. If nothing appears to happen, or if `^P` is echoed, command line editing isn't available; you'll only be able to use backspace to remove characters from the current line.\n",
    "\n",
    "The interpreter operates somewhat like the Unix shell: when called with standard input connected to a tty device, it reads and executes commands interactively; when called with a file name argument or with a file as standard input, it reads and executes a *script* from that file.\n",
    "\n",
    "A second way of starting the interpreter is `python -c command [arg] ...`, which executes the statement(s) in *command*, analogous to the shell's -c option. Since Python statements often contain spaces or other characters that are special to the shell, it is usually advised to quote *command* in its entirety with single quotes.\n",
    "\n",
    "Some Python modules are also useful as scripts. These can be invoked using `python -m module [arg] ...`, which executes the source file for *module* as if you had spelled out its full name on the command line.\n",
    "\n",
    "When a script file is used, it is sometimes useful to be able to run the script and enter interactive mode afterwards. This can be done by passing -i before the script.\n",
    "\n",
    "All command line options are described in [using-on-general](using-on-general.ipynb).\n",
    "\n",
    "### Argument Passing\n",
    "\n",
    "When known to the interpreter, the script name and additional arguments thereafter are turned into a list of strings and assigned to the `argv` variable in the `sys` module. You can access this list by executing `import sys`. The length of the list is at least one; when no script and no arguments are given, `sys.argv[0]` is an empty string. When the script name is given as `'-'` (meaning standard input), `sys.argv[0]` is set to `'-'`. When -c *command* is used, `sys.argv[0]` is set to `'-c'`. When -m *module* is used, `sys.argv[0]` is set to the full name of the located module. Options found after -c *command* or -m *module* are not consumed by the Python interpreter's option processing but left in `sys.argv` for the command or module to handle.\n",
    "\n",
    "### Interactive Mode\n",
    "\n",
    "When commands are read from a tty, the interpreter is said to be in *interactive mode*. In this mode it prompts for the next command with the *primary prompt*, usually three greater-than signs (`>>>`); for continuation lines it prompts with the *secondary prompt*, by default three dots (`...`). The interpreter prints a welcome message stating its version number and a copyright notice before printing the first prompt:"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "$ python3.8\n",
    "Python 3.8 (default, Sep 16 2015, 09:25:04)\n",
    "[GCC 4.8.2] on linux\n",
    "Type \"help\", \"copyright\", \"credits\" or \"license\" for more information.\n",
    ">>>"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "Continuation lines are needed when entering a multi-line construct. As an example, take a look at this if statement:"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "Be careful not to fall off!"
      ]
     },
     "execution_count": 1,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "the_world_is_flat = True\n",
    "if the_world_is_flat:\n",
    "    print(\"Be careful not to fall off!\")"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "For more on interactive mode, see [tut-interac](tut-interac.ipynb).\n",
    "\n",
    "## The Interpreter and Its Environment\n",
    "\n",
    "### Source Code Encoding\n",
    "\n",
    "By default, Python source files are treated as encoded in UTF-8. In that encoding, characters of most languages in the world can be used simultaneously in string literals, identifiers and comments --- although the standard library only uses ASCII characters for identifiers, a convention that any portable code should follow. To display all these characters properly, your editor must recognize that the file is UTF-8, and it must use a font that supports all the characters in the file.\n",
    "\n",
    "To declare an encoding other than the default one, a special comment line should be added as the *first* line of the file. The syntax is as follows:"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "# -*- coding: encoding -*-"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "where *encoding* is one of the valid codecs supported by Python.\n",
    "\n",
    "For example, to declare that Windows-1252 encoding is to be used, the first line of your source code file should be:"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "# -*- coding: cp1252 -*-"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "One exception to the *first line* rule is when the source code starts with a UNIX \"shebang\" line &lt;tut-scripts&gt;. In this case, the encoding declaration should be added as the second line of the file. For example:"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "#!/usr/bin/env python3\n",
    "# -*- coding: cp1252 -*-"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "**Footnotes**\n",
    "\n",
    "[1] On Unix, the Python 3.x interpreter is by default not installed with the executable named `python`, so that it does not conflict with a simultaneously installed Python 2.x executable."
   ]
  }
 ],
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